It should be in Reverse domain name notation. Do you own a domain? Then it's easy: just revert the order of the parts of the domain, than tack on a component that is specific for your app. E.g. If you would own example.com, you could use com.example.myapp for your app named "My App".

If you don't have a domain, you wouldn't be able to submit the app to the App Store. But for your own personal use, the identifier should not matter much, as long as it is unique. I would probably use com.example.<my name>.<my app>